Hi muttiwilson, in case it is helpful - a few comments ..
your current 'landline' attached to ATA <>gateway - does not support SMS - period.. So your options would be
1) get a new DID with a SIP Trunk supplier who has SMS support, and publish that as the 'way to text us' to your clients. Setup normal SMS config with this SIP Trunk carrier <> linked to your new SIP DID with SMS support - all good.
or
2) port your legacy landline DID over to SIP Trunk / with voip carrier who supports this calling region you need / DID portability to bring your number over to SIP. Then ensure to use SMS suitably configured - and - again you are all set.
ie, it kind of depends how important the SMS functionality is
if you have ability to port your DID to a SIP/VOIP carrier
and how motivated you are to make the change / since it involves some disruption to the current config.
Tim