This error is occurring mainly because you are converting unicode to encoded bytes using str, so to solve the problem you need to stop str and instead use .encode() to properly encode the strings.
It returns a encoded version of a string as a bytes object.
Here, an error is set to specify different error handling scheme and by default, the error is set to “strict” which means that the encoding error raises a UnicodeError, the default encoding is set to “utf-8”.
Using .encode() in your code:
p.customer_info = u' '.join((customer_phone, customer_telno)).encode('utf-8').strip()