If I was an employer and your current employer's came up on caller ID when I was calling in response to an application or lead I received about you, I wouldn't hesitate to hang up the phone.
From an ethical perspective, it demonstrates your willingness to exploit your current employer to further your career. It may sound a little extreme, but it's a little more bold and brazen than I would expect.
- Use company resources (computer, printer, envelopes) to draft your resume? You wouldn't be the first, by a long shot.
- Take a long lunch hour to go to an interview? It may require a little imagination, but it's been done.
- Use the company phone system or company-issued cellphone to call recruiters or potential employers? Ethically muddy waters, but not out of the ordinary.
In your case, you're actually using your company cell phone to receive calls to find other employment. That's like cheating on your girlfriend, but giving your mistress your girlfriend's nightie to wear because it really gets your motor running. Even if your girlfriend never finds out, the mistress already knows what you are capable of doing. Initially, she may feel special and flattered because of the risks you're taking to be at your best with her, but, eventually, she'll figure out, based on your history, she probably needs to take regular inventory of her underwear drawer.
You're going to need to weigh the costs of getting your own cell phone against the potential benefit of the new job you're looking for. If you can't afford to get a basic personal cell phone used simply for the purpose of allowing potential employers to contact you, then you may need to stick it out on your current job until you can afford to look for a new one.