[DTMF dial pad] fix unexpected string behavior

[dmceachernmsft]

What

Fix behavior of dtmf dialpad so end users dont lose track of last tone entered. https://user-images.githubusercontent.com/94866715/197297184-d441e0f0-92ac-4681-b642-c7f355f6dfcd.mp4

Why

inputs were being lost after certain characters were entered on the dialpad

https://skype.visualstudio.com/SPOOL/_workitems/edit/3015850

How Tested

tested locally to verify correct behavior

[github-actions[bot]]

Storybook URL https://60c7ae6891f0e90039d7cd54-cydckehqdq.chromatic.com/

[github-actions[bot]]

Calling bundle size is increased❗.

• Current size: 5268480
• Base size: 5268470
• Diff size: 10

[github-actions[bot]]

CallWithChat bundle size is increased❗.

• Current size: 5655592
• Base size: 5655582
• Diff size: 10

[github-actions[bot]]

Chat bundle size is not changed.

• Current size: 5385946
• Base size: 5385946
• Diff size: 0

[github-actions[bot]]

Storybook URL https://60c7ae6891f0e90039d7cd54-zzbbfbnmrh.chromatic.com/

[github-actions[bot]]

Storybook URL https://60c7ae6891f0e90039d7cd54-mlvlhbujhb.chromatic.com/

[github-actions[bot]]

Storybook URL https://60c7ae6891f0e90039d7cd54-whscuocbov.chromatic.com/

[github-actions[bot]]

Storybook URL https://60c7ae6891f0e90039d7cd54-xlrdtcpezf.chromatic.com/

[github-actions[bot]]

Storybook URL https://60c7ae6891f0e90039d7cd54-ztmmthssfj.chromatic.com/

[github-actions[bot]]

Storybook URL https://60c7ae6891f0e90039d7cd54-lhttlyfbjp.chromatic.com/

[github-actions[bot]]

Storybook URL https://60c7ae6891f0e90039d7cd54-yxwaqizate.chromatic.com/