Your PBX uses Feature Codes that begin with *1 or has Extension Numbers that begin 1xx that you cannot dial.
Microsoft have advised us that (ignoring the leading *), Teams is restricted from dialling numbers commencing 1 and generally, have also advised the avoidance of numbers that are less than 4 digits long.
The reason, they advise, is to avoid conflicts with Emergency or Reserved Numbers used for publicly available services.
Whilst we acknowledge that, we are aware that 3-digit extensions numbers can be dialled if they do NOT conflict with emergency services. Microsoft, however, have advised against this as future plans to use these number lengths globally for features, cannot be ruled out.
What Do I Do?
You can create number translation plans in your PBX to simply convert numbers or expand your feature codes by adding new codes that Teams will accept.