Mazhar Hussain, 38, admitted having sexual activity with the teenager after she made contact with West Midlands Police having discussed the grooming with a family friend.
The teenager explained to officers that she believed she had entered into a marriage with Hussain after an over-the-phone ceremony.
The girl had been given that phone, plus a bracelet, as birthday gifts by Hussain.
Detectives found 2,244 message exchanges between him and the girl, many of which were explicit, all sent over a five-week period in 2018.
The messages also showed that Hussain had met with the girl and that sexual activity had taken place.
On a number of occasions Hussain and the girl referred to each other as husband and wife.
Hussain, of Old Farm Road, Stechford, Birmingham, admitted four counts of sexual activity with a child under the age of 16 at Birmingham Crown Court.
On Monday he was sentenced to three years and three months in jail.
He was also put on the sex offenders register and given a sexual harm prevention order for life.
Det Con Dave Cooper, from West Midlands Police's public protection unit, said: "We must commend this girl for having the courage to tell us about what was happening to her.
"We were able to support her with specially trained officers as the case progressed and her evidence proved crucial in ensuring Hussain has now been jailed.
"We also hope that this sentencing will encourage other youngsters who may be being groomed to be brave enough to come forward and let us help and support them too."