How to retrieve "Contact Key" in a text response

Question

I have a list of contacts from the Sales Cloud that I have imported into a MobileConnect list. I stored the Id of a Person Account record from Sales Cloud as the "Contact Key". Here's the CSV file that I have used to import the contact into the MobileConnect list.

_MobileNumber,_LastName,_FirstName,_Email,_CountryCode,_ContactKey 91990007913,Smith,Jane,jane@smith.com.au,AU,0019000003edTguAAE

I mapped the _ContactKey column to the "Contact Key" field on the MobileConnect list. The contact was imported and I can see that the Id from the Person Account record of the Sales Cloud is now the Contact Key in the Marketing Cloud.

I have set up a campaign whereupon receiving a keyword from the subscriber, I want to update a specific field in the Sales Cloud Contact record.

I have used the following lines of code in my Text Response message associated with the keyword. From the various articles and stackexchange questions and answers, I gather that the Contact Key is the SubscriberKey

Here's the text representation of the code. I couldn't get the formatting right. When I try sending an Outbound message and responding with the TESTBED keyword, all I get in response is "There was an error processing your message.". I'm not sure what I am doing wrong. Any help would be much appreciated.

%%[ 
var @response, @contactId, @subscriberKey, @result 

if trim(uppercase([MSG(0).NOUN(0)])) == "TESTBED" 
    set @contactId = Lookup("_MobileAddress","_ContactID","_MobileNumber",mobile_number) 

    set @subscriberKey = Lookup("_Subscribers","SubscriberKey","SubscriberID",@contactId) 

    set @result = UpdateSingleSalesforceObject("Account",@subscriberKey,"Opt_Out__c","true") 

    set @response = CONCAT("TESTBED message received. Thank you.", " Opt Out Result is ", @result) 
else 
    set @response = "Sorry, we did not recognise your response." 
]%% 

%%=v(@response)=%%

Answer 1

After speaking with Marketing Cloud support, I was advised to store the Salesforce Id for a Contact or Account in a custom field (SF_ID of type text) in the Mobile Demographics.

I was able to make use of the _MobileAddress data view to get the value from the SF_ID custom field. Also, while importing contacts in MobileConnect, I had to map the Salesforce Id field to the SF_ID custom field.

Here's the final code that worked for me:

%%[
var @response, @contactId, @subscriberKey, @result 

if trim(uppercase([MSG(0).VERB])) == "TESTBED" then
    set @subscriberKey = Lookup("_MobileAddress","SF_ID","_MobileNumber",MOBILE_NUMBER)
    set @response = CONCAT("Message received. You have been opted out.)
    set @result = UpdateSingleSalesforceObject("Account",@subscriberKey,"Opt_Out__c","true")
else
    set @response = CONCAT("Sorry, we did not recognise your response.", " NOUN value is: ", uppercase([MSG(0).VERB]))
endif
]%%

%%=v(@response)=%%