Hi Steve,
Upon investigation, the AdWords API only limits up to 35 characters of input for the descriptions of call-only ads and returns an error if exceeded. However, upon checking AdWords UI, AdWords UI is implemented with an 80 character limit for descriptions.
Unfortunately, we could not answer as why it is only limited to 35 characters and if you think your description will exceed to over 35 characters, you can do it in AdWords UI for the mean time. Let me know if you have any more concerns.
You can follow our
blog for any future updates on this.
Regards,
Dannison
AdWords API Team