Hi Andriy,
SAP TM101 means TM 8.1. In 8.1 we provide end-2-end integration with SMC3, you just need to maintain this calculation method in your TCCS and it will start working. I'm not sure if you are trying you use SMC3 or some other external service providers.
BRF+ wiil not help if you are trying to invoke a web service call for external charge calculation, but it gives a possibility to do complete charge calculation in BRF+ using formula & expressions.
Just implementing /SCMTMS/IF_TC_CALC_METHODS interface will not suffice, i can help you if you share more details of the web service etc.
Thanks & Regards,
Tarun Kumar