Problem with phone number based login with OTP verification

Hei there,
I want to ad a login which uses phone number to send OTP and verify the phone number. currently I am using API of ‘https://www.getapistack.com/’. But it seems a bit problem occuring with their API. so is there any other API or third party which I can use to set my OTP based phone number verification… If there any tutorial available for that.
And my second question is I use page variable phonenumber as number but the country code contains a “+” sign. its of text type… what should I do ??